This invention relates to partially stabilized zirconia and its application to an electrochemical device. It is especially related to partially stabilized zirconia containing yttrium oxide with improved thermal shock resistance and its application to an electrochemical sensor.
During a cooling process after firing in its preparation, zirconia has been known to undergo a phase transition from a cubic system in high temperatures to a tetragonal system and another phase transition from the tetragonal system to a monoclinic system, which is stable at room temperature. A significant volume change is involved in the phase transition from the tetragonal system to the monoclinic system or vice versa, and this volume change tends to lead to cracks in a fired body of zirconia.
Partially stabilized zirconia containing a stabilizer such as yttrium oxide, magnesium oxide, calcium oxide, etc. has been developed to stabilize a cubic phase and/or a tetragonal phase in zirconia such that these phases are maintained as meta-stable phases down to room temperature without the phase transition to the monoclinic system. Such partially stabilized zirconia has been well known to possess high mechanical strength and strong toughness.
Due to its application to various fields such as heat resistant materials and members, solid electrolyte bodies, etc., partially stabilized zirconia is required to possess not only satisfactory mechanical strength but also satisfactory thermal shock resistance at high temperatures.
Partially stabilized zirconia, however, has not yet attained satisfactory thermal shock resistance. Thermal shock resistance can be parameterized by a critical temperature difference beyond which a sample breaks or cracks because of a sudden temperature change. Usually this bearable limit of a sudden temperature change is tested by quenching a hot sample by immersing it in water at room temperature. For example, according to Somiya; Yoshimura "Zirconia Ceramics" Uchida-Roukakuho:Tokyo; 1989; pp 221-230, partially stabilized zirconia, mainly composed of grains belonging to a cubic system, which contains 6% by mole of yttrium oxide, has a critical temperature difference of 200.degree. C., and partially stabilized zirconia, mainly composed of a tetragonal system, which contains 2 to 3% by mole of yttrium oxide, has a critical temperature difference of 250.degree. C. to 275.degree. C. These values are not large enough in many applications of partially stabilized zirconia.
There has not been much research on improving thermal shock resistance of partially stabilized zirconia with notable results. One of the examples is found in "International Symposium on Science and Technology of Sintering", Fourth Ed.: Tokyo; 1988; pp 1155-1160. This example has provided a method of obtaining partially stabilized zirconia containing 20% by volume of alumina and another method of obtaining partially stabilized zirconia containing 10% by volume of mullite. Additionally, a method has been disclosed in Japanese Patent Laid-Open No. 56-41873 of obtaining zirconia which is mainly composed of grains belonging to a cubic system and which contains 0.5 to 4.5% by weight of grains of a monoclinic system.
None of the three methods is satisfactory. The method of obtaining partially stabilized zirconia containing 204 by volume of alumina, requires temperatures higher than 1600.degree. C. in firing or an HIP treatment because a considerable amount of alumina is added. This temperature requirement leads to a higher cost and limitations in its application. Moreover, firing at such high temperatures may help grains belonging to a tetragonal system grow to result in decreased thermal stability of a product. Furthermore, a large content of alumina may lead to a product without satisfactory oxygen ion conductivity when applied to a solid electrolyte body.
Similarly, the method of obtaining partially stabilized zirconia containing 10% by volume of mullite, has all the disadvantages mentioned above. Moreover, mechanical strength of the zirconia worsens due to incorporation of so much mullite.
Finally the method disclosed in Japanese Patent Laid-Open No. 41873 (1981), includes a step of providing heat hysteresis by carefully controlling temperatures during and after firing, and is useful in obtaining partially stabilized zirconia containing Y.sub.2 O.sub.3, CaO, and MgO with high thermal shock resistance and durability for long usage.
However, the critical temperature difference of this improved partially stabilized zirconia is still about 250.degree. C., and its bending strength is half that of partially stabilized zirconia containing yttrium oxide in general. Moreover, there remains a potential problem of decreased oxygen ion conductivity due to inclusion of zirconia grains having a monoclinic system.